Preparation

Pull off legs from shrimp. Rub most of onion mixture under shells of shrimp. Arrange shrimp in single layer on large baking sheet. Add mushrooms to remaining onion mixture in bowl; toss to coat. Arrange on same baking sheet. Roast shrimp until opaque in center and mushrooms until tender, about 7 minutes. Divide between 2 plates.

MORE INFO:
If you can't find deveined shrimp, use scissors to cut along the back side of each shrimp, exposing the vein. Remove the vein, leaving the shell intact.